You can still use Percona XtraBackup 2.0 for MySQL 5.0 or MySQL 5.1 with built-in InnoDB. There is actually a bug redirecting from 2.0 docs to 2.1 docs that is already reported. But you can download PDF docs of Xtrabackup 2.0.
Also 2.0 version is still actively developed, see last release notes: http://www.percona.com/doc/percona-x...2.0/2.0.7.html